## Query Ops

This crate provides a small set of query operators that can be used to implement a simple query-plan executor.
The operators are: atom scan, semijoin, and natural join.

### Public API

| Item                                             | Type     | Description                                                                                                                                                                                                         |
|--------------------------------------------------|----------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `scan_atom(&Table, &AtomPattern) -> Relation`    | function | Scans the table under the pattern and returns a binding relation with one column per distinct variable in first-occurrence order. Literal positions and repeated variables filter rows during the scan.             |
| `semijoin(&Relation, &Relation) -> Relation`     | function | Returns the rows of `left` whose values on the columns shared with `right` also appear in `right`. The output column list is the same as `left.columns`.                                                            |
| `natural_join(&Relation, &Relation) -> Relation` | function | Returns every pair of `left` and `right` rows that agree on shared columns. Each output row holds the columns of `left` followed by the non-shared columns of `right`.                                              |
| `Table`                                          | struct   | Holds positional input rows of fixed arity and carries no column names. Construct it with `Table::new(arity)` or `Table::from_rows(arity, rows)`.                                                                   |
| `AtomPattern`                                    | struct   | Specifies, for each table column, either a variable to bind or a literal value to match. The pattern is a `Vec<Term>` whose length must equal the table's arity.                                                    |
| `Term`                                           | enum     | Represents one position of an `AtomPattern`. A term is either `Var(String)` to bind the cell to a named variable, or `Lit(Value)` to require the cell to equal a given value.                                       |
| `Relation`                                       | struct   | Holds rows over named columns and is the type produced by every operator. Construct it with `Relation::new(columns)` or `Relation::from_rows(columns, rows)`. Column names within a single relation must be unique. |
| `Value`                                          | enum     | Represents a single cell value stored in a `Table` or `Relation`. A value is either `Int(i64)` or `Str(String)`.                                                                                                    |

### Example

`Q(X) :- edge(X, X), labeled(X).` (labeled self-loops):

```rust
use query_ops::atom::{AtomPattern, Term, scan_atom};
use query_ops::join::semijoin;
use query_ops::table::Table;
use query_ops::value::Value;

fn main() {
    let edge = Table::from_rows(
        2,
        vec![
            vec![Value::Int(1), Value::Int(2)],
            vec![Value::Int(3), Value::Int(3)], // self-loop on 3
            vec![Value::Int(2), Value::Int(2)], // self-loop on 2
        ],
    );
    let labeled = Table::from_rows(1, vec![vec![Value::Int(2)]]);

    let self_loops = scan_atom(
        &edge,
        &AtomPattern {
            columns: vec![Term::Var("X".to_string()), Term::Var("X".to_string())],
        },
    );
    let labeled_x = scan_atom(
        &labeled,
        &AtomPattern {
            columns: vec![Term::Var("X".to_string())],
        },
    );
    let result = semijoin(&self_loops, &labeled_x);

    assert_eq!(result.columns, vec!["X".to_string()]);
    assert_eq!(result.rows, vec![vec![Value::Int(2)]]);
}
```

### Test

```sh
cargo test -p query-ops
```